They're a soil problem, not a plant problem
Those small flies hovering near your plants aren't actually interested in the plant itself. They're living in the top inch or two of wet potting soil, where their larvae feed on organic matter and fungus. Change the conditions in that top layer and the gnats lose their nursery, whether or not you ever spray a single leaf.
Let the surface dry out
This is the single most effective fix. Fungus gnat larvae can't survive in dry soil, so let the top inch dry completely between waterings. Check with a finger before you water, not on a fixed schedule. For plants that like to stay evenly moist, water from the bottom instead of pouring water over the top, which keeps the surface drier while the roots still get what they need.
Dry topsoil is quiet soil. Gnats need moisture to breed, and the top inch is where they do it.
A few things that speed it up
- A thin layer of sand, fine gravel or diatomaceous earth on top of the soil makes it harder for adults to lay eggs there.
- Yellow sticky traps near the pots catch the adults and let you see whether the population is actually shrinking.
- A hydrogen peroxide drench (one part 3% peroxide to four parts water, watered in like normal) kills larvae in the soil without harming most houseplants.
If it's not clearing up
A heavy, ongoing infestation usually means the soil is staying wetter than the plant needs, sometimes because the pot has no drainage, sometimes because the mix itself is too dense. Bring the plant in and we can take a look at both the pest and the setup underneath it.
